I sat the old sankyuu (same as current N4) in 2007-ish, and my Japanese then was nowhere near then as good as yours is now. Still got something like 90%.

I'd like to say that you could pass N2 in a couple of years, as I really don't think your Japanese will be any worse then than mine was when I passed the nikyuu. But it can be difficult to gauge someone's ability from the odd conversation online and the Japanese LPs they've dabbled in. :'D Still, give it consideration. I don't think it's beyond you at all.
